The present invention is directed to a circuit apparatus for measuring the acoustic admittance of the ear.
In the practice of tympanometry, measurements of the electrical impedance of the tympanic membrane of the inner ear (actually the admittance thereof, which is the reciprocal of the impedance) are made as ranges of air pressure or vacuum levels are applied thereto in the presence of tone signals of single, or more recently differing, frequency. The positive and negative pressure levels are applied to the eardrum via a probe inserted into the ear which also serves to apply the acoustic tone levels and act as a microphone to measure the admittance of the tympanic membrane in response to the tone stimulus as pressure changes are made. A number of tympanometry testing instruments are commercially available to the practitioner; including those made by Amplaid Spa, of Milan, Italy, Grason-Stadler, of Littleton, Mass., and Madsen Electronics, of Oakville, Ontario and Buffalo, N.Y. However, at present, the existing tympanometric instrumentation in the market allows only a few frequency tones to be applied to the eardrum, typically the standard frequency tones of 226 and 678Hz. To expand the range of diagnostic evaluation in this field it is desirable to perform multiple frequency tympanometric measurements and thus to be able to apply to the ear cavity via a probe selected tones at various frequencies over the mid-sonic range. During tympanometry, it is desirable to have a fast response to changes in the ear cavity's admittance in order to reduce the total measurement time and to track the changes in admittance. On the other hand, in another type of test, when a stimulus tone signal is present together with a standard probe tone level signal in measuring the acoustic reflex response of the ear, it is desirable that the bandpass filter have a narrow bandwidth in order to better reject large amplitude stimulus signal which may be up to 40 dB above the probe tone level.